The present invention relates generally to interferometry and in particular to improved performance of interferometry by the patterning or otherwise shaping of one or more beams in an interferometric device.
Precision laser interferometry can be used to precisely determine (or monitor changes of) the distance to one or more fiducial points, such as a flat mirror, rooftop mirror, or corner-cube retro-reflector (“retro”), or between such fiducial points. An interferometer generally is composed of three components or subsystems: (1) a radiation source (e.g., a laser device or some other suitable beam generator), (2) an optics component for producing beams of light for reference, measurement, and so on (also referred to as a “beam launcher”), and (3) a signal processor (e.g., an observer or a photo-detector and associated electronic circuits) or other processing component to perform the interferometric determinations. In some configurations, the photo-detector is included in the optical component (beam launcher), while much of the supporting electronics (e.g., the phase meter(s) and computer) remain with the signal processor. As can be appreciated other subsystem configurations are possible.
Interferometers can be configured to operate in a number of ways. The present invention is applicable to optical interferometers in general, operating in the regions of the electromagnetic spectrum commonly referred to as the infra-red (IR) light region, visible light region, and ultra-violet (UV) light region. Since there are many configurations of optical interferometers, only a small sampling of interferometer configurations will be discussed for background purposes. It will therefore be understood that a “beam” in the context of the present invention can be IR, visible light, or UV.
Some interferometers monitor the distance between two retros by directing a single beam towards a first one of the retros. The single beam hits the first retro at a point that is offset from a vertex of the retro. The retro-reflected beam emerges from the first retro at a symmetrically located offset point, and the beam then is directed to a second retro. The beam and retros are positioned and aligned such the reflected beam hits the second retro also offset from the vertex, with the emerging beam doubly reflected back to an entrance point on the launcher. Such a circuitous configuration is sometimes referred to as a “racetrack” configuration.
Precision laser interferometry can be carried out in at least two modes, namely, the “homodyne” mode or the “heterodyne” mode. Either mode can be used for either the racetrack configuration.
In the homodyne mode, a beam launcher splits a laser beam of a single frequency into two beams. One beam is directed out to the fiducial(s) to measure the distance. Upon returning to the beam launcher, the beam is aligned and collocated (and the polarization aligned, if needed) with the other portion of the original beam, and the resulting combined beam is directed onto a photo-detector. If the extra distance traveled by the measurement beam is an integer multiple of half the laser wavelength, then, when recombined, the two beams are in phase and add constructively, resulting in an increased signal from the photo-detector. If the measurement beam is an odd multiple of a quarter of the wavelength longer, the beams add destructively, resulting in a reduced signal from the photo-detector. If the distance between the retros changes, the signal fluctuates, and the fluctuations in the signal give a measure of the relative motion of the retros. A signal processor (e.g., an observer or a photo-detector and electronic circuit) “counts fringes” to determine the change in distance between the retros relative to an initial distance. The resolution of a homodyne interferometer is limited, as it is difficult to measure changes in distance significantly smaller than the laser wavelength (typically a half to several micrometers) due to intensity fluctuations of the laser.
A heterodyne interferometer configuration uses two beams that are offset in frequency to slightly different frequencies. Typically, the beams originate from a single laser. The difference between the frequencies is chosen to be convenient for detectors and electronics. Typically, the frequency difference is in the range of about 10 kHz to about 100 MHz. Typically, one frequency-offset laser beam (the “measurement beam”) emanates from the beam launcher to interrogate the distance to the retro(s) while the second frequency-offset laser beam (the “local oscillator” or LO) beam remains internal to the beam launcher. When the measurement beam and the LO beam are aligned, collocated, and with aligned polarizations, and are directed onto the photo-detector, the photo-detector produces a “beat” signal. By comparing this beat signal to the known difference of frequency offsets between the laser beams, it is possible to track changes in the relative phase of the signal to find the change in retro distance relative to the initial value. With precision phase meters, it is possible to resolve distances to small fractions of the laser wavelength, resulting in measurements with sub-nanometer precision.
When measuring distances with fine precision, various error sources can affect the results. The laser intensity can fluctuate. The laser radiation is often routed to the beam launcher by means of optical fibers, where small effects such as a temperature variation or a strain on the fiber can affect the apparent optical length of the fiber and can result in a phase change that erroneously appears to be a measured displacement of the fiducial points. These errors can be reduced by replacing the “known difference” of the laser frequency offsets with a “reference signal” that measures the frequency difference directly. This reference signal is created by mixing a portion of the LO beam with the “reference beam”, which is a portion of the first laser beam that does not interrogate the distance between retros, and directing the combined beam onto a second photo-detector. The use of a reference beam significantly reduces the errors introduced by any common element (e.g., laser or fiber), but it cannot correct for elements that are unique to the measurement path or the reference path. Other errors can be reduced by sharing elements between the measurement and LO beams. The measurements are not affected by elements in the beam-path “downstream” from the point where the two laser beams are first combined (the point where they become aligned, like-polarized, and collocated), as the elements are common to both beams.
In the optical component referred to as the beam launcher, the various beams of light are bent in different directions (e.g., by the use of reflecting surface such as mirrors) and made to pass through various openings formed in the beam launcher. The diffraction of light that results when a wide beam of light passes through an opening that is narrower than the beam width is generally an undesirable artifact. When such a diffracted beam propagates a distance, the fringing effect becomes more pronounced. These optical artifacts may result in cross-talk if light rays from different beams go to the wrong detector channels, thus giving rise to “cyclic error” that can result in erroneous distance determinations. It is possible to model these diffractive affects and to introduce masks that reduce the effect, as disclosed in U.S. application Ser. No. 10/293,209, for example. However, it may be necessary to mask out more of the beam than is desirable from the point of view of laser power conservation.
Additionally, a beam may hit an optic surface at its edge (e.g., a hole in a separation mirror). It is difficult to maintain adequate mirror surface quality immediately adjacent to an edge. There are often chips in the surface, bevels, or other surface irregularities. These artifacts can also result in diffraction artifacts.